The present invention generally relates to an apparatus for detecting faults and more particularly, to a fault detecting apparatus for detecting faults in transparent bodies or objects such as glass bottles and the like.
As apparatuses for effecting fault detection of the above described type, there have conventionally been known one type which adopts the dark-on system, and the other type which is based on the polarized light-on system.
The dark-on system as referred to above is the system arranged to project dark portions corresponding to foreign matters and the like, onto a detecting plane through utilization of a light shielding property of such foreign matters mixed in a raw material of a transparent object to be inspected.
However, in this dark-on system, the items to be inspected are limited to light shielding foreign matters such as pieces of bricks or metals, etc. For example, in the case where the item, to be inspected is a glass bottle, mixing of a foreign matter such as finely crystallized glass material, into the raw material is not permitted due to formation of undesirable strains or distortion, but in the dark-on system as described above, it is impossible to detect such transparent foreign matters. Moreover, in this system, if it is intended to raise the sensitivity so that even very small foreign matters may be detected, seam, satin-like finish, engraving, etc. which should not be regarded as foreign matters, are undesirably detected without being differentiated from the foreign matters, and thus the object is wrongly judged as a faulty item. The arrangements based on the above system are conventionally disclosed, for example, in U.S. Pat. Nos. 3,727,068 and 4,280,624.
On the other hand, the polarized light-on system is the system in which polarizing plates are respectively disposed at a light incident side where light is projected onto the object to be inspected, and also, at a detecting side where light transmitted through the object to be inspected is detected, with main axes of these polarizing plates being set to intersect at right angles to each other (as disclosed in U.S. Pat. No. 3,963,348). In the case where no foreign matters and the like are present in the raw material of the object to be inspected, since the plane polarized light taken out from the incident side polarizing plate is transmitted as it is through the object to be inspected, this transmitted light is shielded by the detecting side polarizing plate so as not to reach the detecting plane. Meanwhile, in the case where foreign matters are present in the raw material to form strains thereat, a plane polarized light component transmitting through the detecting side polarizing plate is formed in the light transmitted through the object to be inspected, by the photoelastic effect at the above strain portion. By detecting such a component on the detecting plane, detection of presence of foreign matters, i.e. the fault detection is to be effected.
However, most of the foreign matters mixed in the raw material are generally free from strains even if they are large in size, and moreover, such foreign matters have a light shielding property in most cases. Accordingly, transmitted light corresponding to the foreign matters can not be detected in almost all cases, and only the transmitted light formed at the peripheral portions of the foreign matters due to strains may be detected somehow. Therefore, the detecting accuracy of the polarized light-on system as described so far is considerably low as compared with that of the dark-on system. If it is intended to raise the detecting accuracy by the polarized light-on system, an expensive photoelectric detector must be employed, with a further necessity for increasing light intensity of a light source, and therefore, it is required to take various countermeasures such as installation of a cooling device to prevent the polarizing plate from melting by heat due to light or provision of a water distribution system, thus resulting in such disadvantages the constructions are undesirably complicated, with consequent inconvenience in handling and difficulty in maintenance, etc.
Apart from the foregoing arrangements, as one example of the dark-on system, it may be so arranged that a pair of plane polarizing plates whose main axes are directed in the same direction, are disposed at front and rear sides of the object to be inspected so that light can be transmitted up to the detecting side only when no foreign matters are present in the object to be inspected, for detection of faults in the transparent bodies. In the above case, however, troublesome procedures are required for the setting to align the main axes of the plane polarizing plates, while any sufficiently favorable result has not been available thereby, either.